Not Applicable
Not Applicable
The present invention relates to digital radiography, and more particularly, to digital radiography signal processing techniques for improving signal to noise ratios of an image generated from a bi-chromatic x-ray beam.
X-ray images of non-homogeneous material simultaneously display areas of different attenuation. In some cases, the attenuation differences are significant. For example, chest images simultaneously display areas of great attenuation (e.g., mediastine, spine) and areas of little attenuation (e.g., lungs). Optimum x-ray imaging of these two differing anatomical structures requires different parameters of x-ray flux. Specifically, areas of higher attenuation are better visualized by utilizing lower energy photons, whereas areas of lower attenuation are better visualized by utilizing higher energy photons. For this reason, chest images are generally taken with x-ray photons having a broad energy spectrum or, as it is done in certain procedures recently developed, heat images are taken with two narrow band x-ray pulses that have different average energies.
In both cases, the signal to noise ratio of the images so obtained is lower than the signal to noise ratio of an image obtained with a single narrow band x-ray pulse of the same dosage. The reason for the decreased signal to noise ratio is related to the fact that x-ray photons of different energies generate a different average amount of charge. As a result, the variance of the measured charge signal is affected by both the statistical variance of the photons, and the variance of the charge generated of them. This is known as xe2x80x9cSchwank effect.xe2x80x9d
The following description quantifies the decrease of the signal to noise ratio of an image taken with a bichromatic (i.e., two energy) x-ray beam, with respect to the signal to noise of an image taken with a monochromatic beam, assuming that the two beams have the same number of x-ray photons.
Monochromatic X-ray Beam: Single Image
In the case of a single image I produced from a monochromatic (i.e., one energy level) x-ray beam, the signal S and noise "sgr" of the image I is given by:
S=xcex1N"sgr"=xcex1{square root over (N)}
(S/"sgr")2=N
where N is the average number of x-ray photons detected (i.e., the average photon flux per unit area), and xcex1 is the average number of electrons generated per photon. The parameter xcex1 is characterized by a single value at least in first approximation, because the energy of the monochromatic x-ray photons has a narrowband spectrum.
Bichromatic Beam: Single Image
The average number of detected x-ray photons N may be partitioned into two groups as follows: N1 photons generate xcex11 carriers/photon and N2 photons generate xcex12 carriers/photon, such that N1+N2=N. The N1 photons are relatively narrowband with a single mean energy E1, and the N2 photons are relatively narrowband with a single mean energy E2, with E1 not equal to E2. The signal S and noise "sgr" of the image I is given by:       S    =                                        α            1                    ⁢                      N            1                          +                              α            2                    ⁢                      N            2                    ⁢                      xe2x80x83                    ⁢          σ                    =                                                  α              1              2                        ⁢                          N              1                                +                                    d              2              2                        ⁢                          N              2                                                              (                  S          /          σ                )            2        =                                        (                                                            α                  1                                ⁢                                  N                  1                                            +                                                α                  2                                ⁢                                  N                  2                                                      )                    2                                                    α              1              2                        ⁢                          N              1                                +                                    α              2              2                        ⁢                          N              2                                          =                                    (                                          N                1                            +                              xN                2                                      )                    2                                      N            1                    +                                    x              2                        ⁢                          N              2                                                      id      .        ,          
        ⁢                  and        ⁢                  xe2x80x83                ⁢        x            =                        α          2                /                  α          1                    
The signal to noise ratio (S/"sgr")2 is a function of x, i.e., (S/"sgr")2=ƒ(x).
For x=1, i.e., xcex11=xcex12, the case 2) reduces to the case 1) in which the x-ray beam has monochromatic energy.
It can also be shown that f(x=1) is a maximum of the function f(x), because                     ⅆ                  f          ⁡                      (            x            )                                      ⅆ        x              =                  0        ⁢                  xe2x80x83                ⁢        for        ⁢                  xe2x80x83                ⁢        x            =      1        ,                    and        ⁢                  xe2x80x83                ⁢                                            ⅆ              2                        ⁢                          f              ⁡                              (                x                )                                                          ⅆ                          x              2                                           less than               0        ⁢                  xe2x80x83                ⁢        for        ⁢                  xe2x80x83                ⁢        x              =    1  
This means that the signal to noise ratio of the monochromatic case is higher than that of a bichromatic case, when the number of photons is equal in both cases.
It can also be shown that in general, the value (S/"sgr")2 for the monochromatic case is greater than the value (S/"sgr")2 for multi-chromatic beams, including the case of a broadband x-ray beam comprising x-ray photons having a broad energy spectrum. This result is known to those in the art as the xe2x80x9cShwank Effect.xe2x80x9d
It is an object of the present invention to substantially overcome the above-identified disadvantages and drawbacks of the prior art.
The foregoing and other objects are achieved by the invention which in one aspect comprises a method of improving the signal to noise ratio associated with the output of a digital x-ray detector receiving bi-chromatic x-ray energy. The method includes acquiring a first image from the detector corresponding to x-ray energy at a first energy level, and scaling the first image with a first scaling factor so as to produce a scaled first image. The method further includes acquiring a second image from the detector corresponding to x-ray energy at a second energy level, and scaling the second image with a second scaling factor so as to produce a second scaled image. The method also includes combining the first scaled image and the second scaled image so as to form a compensated image.
In another embodiment of the invention, the first scaling factor is substantially equal to the product of a ratio and a fixed constant. The ratio is xcex12/xcex11, where xcex12 is the average number of carriers per photon generated by the x-ray energy at the second energy level, and xcex11 is the average number of carriers per photon generated by the x-ray energy at the first energy level. The second scaling factor is substantially equal to the fixed constant.
In another embodiment of the invention, combining the first and second scaled images further includes adding the first scaled image to the second scaled image.
In another aspect, the invention comprises a system for improving the signal to noise ratio associated with the output of a digital x-ray detector receiving bi-chromatic x-ray energy. The system includes a first multiplier for multiplying a first scaling factor by a first image acquired from the detector so as to produce a scaled first image. The first image corresponds to x-ray energy at a first energy level. The system also includes a second multiplier for multiplying a second scaling factor by a second image acquired from the detector so as to produce a scaled second image. The second image corresponds to x-ray energy at a second energy level. The system further includes a combiner for combining the first scaled image with the second scaled image so as to produce a composite image.
In another embodiment of the invention, the first scaling factor is substantially equal to the product of a ratio and a fixed constant. The ratio includes the average number of carriers per photon generated by the x-ray energy at the second energy level, divided by the average number of carriers per photon generated by the x-ray energy at the first energy level. The second scaling factor is substantially equal to the fixed constant.
In another embodiment of the invention, the combiner includes an adder, such that the combiner adds the first scaled image to the second scaled image to produce the composite image.
In another aspect, the invention comprises a system for improving the signal to noise ratio associated with an output of a digital x-ray detector that receives bi-chromatic x-ray energy and produces an electrical signal representative of the x-ray energy. The system includes an x-ray source for generating x-ray energy at a first and second energy level, and directing the x-ray energy through an object and toward the detector, so as to project a two-dimensional image of the object onto a surface of the detector. The system further includes an image processor for controlling the x-ray source to cyclically produce x-ray energy alternating between the first energy level and the second energy level. The image processor also formats the electrical signal from the detector to produce, for each energy level, a block of image data representative of the two-dimensional image of the object. The image processor thus alternately produces blocks of image data representative of the first energy level and the second energy level. The system also includes an image compensator for applying scaling factors to the blocks of image data from the image processor for each energy level, so as to produce scaled blocks of image data, and for combining the scaled blocks of image data to produce a compensated image.
In another embodiment of the invention, the image compensator combines pairs of blocks of image data, each pair of blocks including a first block representative of the first energy level, and a second block representative of the second energy level.
In another embodiment of the invention the image compensator combines the blocks of image data by adding the blocks of image data.
In another embodiment of the invention, the image compensator multiplies a first block of image data representative of a first energy level by a first scaling factor, and multiplies a second block of image data representative of a second energy level by a second scaling factor.
In another embodiment of the invention, the first scaling factor is k(xcex12/xcex11) and the second scaling factor is k. xcex11 is the average number of carriers per photon generated by the x-ray energy at the first energy level, xcex12 is the average number of carriers per photon generated by the x-ray energy at the second energy level, and k is a fixed constant.
In another aspect, the invention comprises a method of improving a signal to noise ratio associated with an output of a digital x-ray detector receiving bi-chromatic x-ray energy and producing an electrical signal representative of the x-ray energy. The method includes generating x-ray energy at a first energy level and at a second level, and directing the x-ray energy through an object and toward the detector, so as to project a two-dimensional image of the object onto a surface of the detector. The method further includes controlling the x-ray source to cyclically produce x-ray energy alternating between the first energy level and the second energy level. The method also includes formatting the electrical signal from the detector to produce, for each energy level, a block of image data representative of the two-dimensional image of the object. The image processor thus alternately produces blocks of image data representative of the first energy level and the second energy level. The method further includes applying scaling factors to the blocks of image data from the image processor for each energy level, so as to produce scaled blocks of image data, and combining the scaled blocks of image data to produce a compensated image.
Another embodiment of the invention further includes combining pairs of blocks of image data, each pair of blocks including a first block representative of the first energy level, and a second block representative of the second energy level.
Another embodiment of the further includes combining the blocks of image data by adding the blocks of image data.
Another embodiment of the invention further includes multiplying a first block of image data representative of a first energy level by a first scaling factor, and multiplying a second block of image data representative of a second energy level by a second scaling factor.
In another embodiment of the invention, the first scaling factor is k(xcex12/xcex11) and the second scaling factor is k. xcex11 is the average number of carriers per photon generated by the x-ray energy at the first energy level, xcex12 is the average number of carriers per photon generated by the x-ray energy at the second energy level, and k is a fixed constant.
In another aspect, the invention comprises a method of improving a signal to noise ratio associated with an output of a digital x-ray detector receiving multi-chromatic x-ray energy. The method includes acquiring two or more x-ray images from the detector, each acquired image corresponding to x-ray energy received at one of two or more energy levels. The method further includes scaling each of the two or more images with an associated scaling factor so as to produce a set of scaled images. The method also includes combining the set of scaled images so as to form a compensated image.
In another embodiment of the invention, combining the set of scaled images includes adding the set of scaled images together.
In another aspect, the invention comprises a system for improving a signal to noise ratio associated with an output of a digital x-ray detector that receives multi-chromatic x-ray energy and produces an electrical signal representative of the x-ray energy. The system includes an x-ray source for generating x-ray energy at two or more energy levels, and directing the x-ray energy through an object and toward the detector, so as to project a two-dimensional image of the object onto a surface of the detector. The system also includes an image processor for controlling the x-ray source to cyclically produce x-ray energy alternating among the two or more energy levels. The image processor also formats the electrical signal from the detector to produce, for each of the two or more energy levels, a block of image data representative of the two-dimensional image of the object. The image processor thus cyclically produces blocks of image data representative of the two or more energy levels. The system further includes an image compensator for applying scaling factors to the blocks of image data from the image processor for each energy level, so as to produce scaled blocks of image data, and for combining the scaled blocks of image data to produce a compensated image.
Another embodiment of the invention, the image compensator combines sets of N blocks of image data, wherein N represents a total number of energy levels produced by the x-ray source.
Another embodiment of the invention, the image compensator combines the blocks of image data by adding the blocks of image data.